2 Answers
2

You cannot equip an assault rifle without a standard weapons skill of 6 in the base game. You might have installed a modification to your game (modifying the gamesys file, shock.gam) to change the requirements.

I know of most of the SS2 glitches, and this is not one I am aware of. I suspect you have a modified version of SS2. There are many balance tweak mods available for SS2, if you obtained your copy from a less-than-legitimate source one could quite possibly be already installed.

I assume that you have modded your System Shock 2 using the detailed modding guide on the GOG webpage. There is a mod among the suggested one which includes a balance patch for SS2. That should be the reason.